Our invention relates to a method of writing one stream of digital data after another on magnetic tape or other record media. The writing method of our invention is of particular utility in conjunction with magnetic tape packaged in cassette form, although we do not wish our invention to be limited to this particular application.
Magnetic tape finds extensive use as a data storage medium of electronic computer systems. Usually, a stream of file data is written in the form of a series of data blocks on a length of magnetic tape. Then two consecutive file mark blocks are created immediately after the last of the series of data blocks. When the tape is read subsequently, the two consecutive file marks serve to inform the host system of the end of one file data stream, and of the fact that the remaining tape length is blank. Thus the unnecessary scanning of the unrecorded tape length is avoided.
In writing another stream of file data on the remaining blank tape length, the usual conventional practice with magnetic tape of the open reel type has been to overwrite the first block of the second file data stream on the second of the two consecutive file mark blocks. Then the second and all the subsequent data blocks of the second file data stream has been created after the first block (FIG. 3). The remaining one file mark between the two file data streams has been used as a demarcation therebetween. The single file mark has enabled the two file data streams to be read continuously.
We object to this conventional practice of overwriting the first block of the second file data stream on the second of the two file mark blocks following the first file data stream. It is no easy task to position the transducer exactly on the second file mark block in writing the second file data stream. Admittedly, this difficulty has been somehow overcome with open reel systems, but not with tape cassette systems. As far as we are aware, there have been no tape cassette systems built on the conventional principle of writing one file data stream after another.
We also know another conventional writing method that employs no file marks. The termination of one file data stream on magnetic tape is sensed according to this second known method when the transducer scans a predetermined length of blank tape portion following the last data block. This scheme is also objectionable because of its susceptibility to wrong operation under the influence of noise.